1. Field of the Invention
The present invention relates generally to hard copy apparatus, more specifically to conductive heating of print media, and particularly to the heating of print media that is advancing through the printing zone of an ink-jet printer.
2. Description of the Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, facsimile machines, and multifunctional peripheral (xe2x80x9cMFPxe2x80x9d) hard copy apparatus employ ink-jet technology for producing hard copy. The basics of this technology are disclosed in various articles in the Hewlett-Packard Journal, for example, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992, and Vol. 45, No. 1 (February 1994) editions. Ink-Jet devices are also describe by W. L. Lloyd and H. T. Taub in Output Hardcopy Devices, chapter 13 (Ed. R.C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
In order to simplify the description of the present invention the term xe2x80x9cpaperxe2x80x9d is used as synonymous with all types of print media; the term xe2x80x9cinkxe2x80x9d is used as synonymous with all compositions of colorant; the term xe2x80x9cpenxe2x80x9d is used as synonymous with all types of ink-jet writing instruments. While the present invention is described for convenience in terms of application to ink-jet printing, it is to be recognized by those skilled in the art that many of the concepts are applicable to any hard copy apparatus using a wet colorant for creating a print. No limitations on the scope of the invention are intended nor should any be implied.
One important factor affecting the print quality of ink-jet printers is drying time. Print media movement must be controlled to ensure that the liquid ink dries properly once printed. Bleed of one color into another can occur when two wet droplets come into contact. Any touching of the printed surface before the ink is dry can result in smearing. An additional concern is paper cockle, which is an uncontrolled, localized warping of the paper that occurs as liquid ink saturates the fibers.
Active heating devices can be and are used to heat a printing sheet in order to speed the drying time. However, heat must be applied carefully to avoid the introduction of other problems. For example, the paper can be scorched. Furthermore, if heat is not applied correctly, the resultant uneven drying time of a color area of an image can produce undesirable variations in the hue characteristics, known as hue shift. Another problem attributable to improperly applying heat is a noticeable warping of the sheet. Normally, paper carries at least some moisture content. For example, a sealed ream of standard office paper has about 4.5-percent moisture content. High ambient humidity can increase the moisture content as the paper sheets lie in an intray. As heat is applied to part of the paper, uneven drying and shrinkage occurs. Uneven shrinkage causes the paper to warp. Some print media, such as polyester-based transparencies, will carry insignificant amounts of water and, therefore, will not buckle a s a result of uneven shrinkage. Such media, however, may warp or even burn if all or portions of it are overheated. Thus, uniform, controlled heating of the media is important for high print quality, irrespective of the type of print media.
A METHOD OF MULTIPLE ZONE HEATING OF INKJET MEDIA USING A SCREEN PLATEN is taught in U.S. Pat. No. 5,668,584 by Broder et al., i s sued Sep. 16, 1997 (assigned to the common assignee herein and incorporated by reference). Pre-printing, print zone, and post-printing heating is shown using an open screen type platen. Other specific methods and apparatus for CONDUCTIVE HEATING OF PRINT MEDIA is described by common inventor Wotton et al., in U.S. patent application Ser. No. 09/1412842, filed Oct. 5, 1999 (xe2x80x9cWotton et al.xe2x80x9d hereinafter), co-pending herewith, assigned to the common assignee herein, and incorporated herein by reference in it s entirety, particularly discussing vacuum holddown type platen technology.
If heat is to be applied to the print sheet, it is useful to have it in the print zone. Heating in the print zone rapidly drives off a substantial portion of the liquid component of the ink so that cockle is unable to form, or at least is minimized. However, when one attempts to heat the media in the print zone, it is important to ensure that the applied heat is not directed into the printhead. If an ink-jet printhead overheats, drop trajectory and other characteristics of the printhead can change, again negatively affecting print quality. Moreover, the heat should not be applied in a manner, such as by convection, that itself may directly alter droplet trajectory.
Another prior art solution is shown by Vincent et al. in U.S. Pat. No. 5,510,822, issued Apr. 23, 1996 for an INK-JET PRINTER WITH HEATED PRINT ZONE (assigned to the common assignee herein and incorporated by reference).
A close study of the thermodynamics of a print zone heater has shown that the problem is more complex than previously thought. Along the x-axis, some of the thermal loads that can cause a temperature imbalance include paper type and size, ink composition and presence or absence (i.e., dotted and not-dotted pixels) in regions of the printing sheet, and airflow such as occurs when using a vacuum-type platen as in Wotton et al. It has been found that airflow near the edge of the printing sheet creates the largest thermal load. This load has been found to create temperatures drops near the edge of as much as 30-degrees Centigrade. Edge-to-edge printing, known as full bleed further exacerbates the problem. At the same time, as a variety of different sized media is usually used in a printer, the edge of the sheet from page to page may be indeterminate. Thus, the load position from airflow near the edge of the paper is a variable factor. Therefore, there is a need for method and apparatus to provide substantially infinitely adjustable power densities along the x-axis in order to ensure a uniform temperature profile.
One solution is to have a very fine heater resolution. However, such is expensive in and of itself and also requires extensive control subsystems.
It has been found that anisotropic thermal conductivity on a heated platen, i.e., having different levels of thermal conductivity in the x-axis, y-axis, and z-axis, provides significant advantages and advancement in the state of the art.
As used herein, the term xe2x80x9chigh thermal conductivityxe2x80x9d shall mean: greater than approximately one-hundred (100) W/m.K (Watts/meter Kelvin). An example of a material having a relatively high thermal conductivity is aluminum.
As used herein, the term xe2x80x9clow thermal conductivityxe2x80x9d shall mean: less than approximately ten (10) W/m.K. An example of a material having relatively low thermal conductivity is plastic.
As used herein, for operating temperatures between about 40xc2x0 C. and 150xc2x0 C., the term xe2x80x9chigh thermal resistancexe2x80x9d shall mean:
in the y-axis,
surface temperature change (xcex94T/LY) xe2x89xa71.0xc2x0 C./mm for surface
temperature changes of up to 90xc2x0 C. between two points; and
in the z-axis,
Powerwatts z-axisxe2x89xa60.15 Powerwatts total.
As used herein, the term xe2x80x9clow thermal resistancexe2x80x9d shall mean: length÷thermal conductivity greater than fifteen percent heat flow (or power flow in Watts).
As used herein, the term xe2x80x9chigh thermal massxe2x80x9d shall mean: a mass having a response time of greater than 60 seconds to change temperature by 100xc2x0 C.; or, as a calculable mxc2x7Cp (mass-specific heat), a specific implementation contemplated by the inventors being mxc2x7Cpxe2x89xa71200 J/K (Joules/Kelvin). A preferred implementation (an example of a component in the current context having a relatively high thermal mass would be the entire platen 42) should have a response time measured in minutes rather than seconds.
As used herein, the term xe2x80x9clow thermal massxe2x80x9d shall mean: a mass having a response time of less than 30 seconds to change temperature by 100xc2x0 C.; as a calculable mxc2x7Cp (mass-specific heat), a specific implementation contemplated by the inventors being mxc2x7Cpxe2x89xa7600 J/K (Joules/ Kelvin), indicative of a response time measured in seconds or fractions of seconds. An example of a component in the current context having a relatively low thermal mass would be the nickel orifice plate of the printhead of a pen 115X.
As used herein, the term xe2x80x9cpower density controlxe2x80x9d shall mean: independent control of power and temperature for various platen areas and having relatively short response times.
As used herein, the term xe2x80x9crapid temperature changexe2x80x9d shall mean: a temperature gradient greater than about one (1) degree Centigrade/millimeter in the paper transit axis, xe2x80x9cy,xe2x80x9d for belt speeds of approximatelyxe2x89xa6one inch per second in a hard copy apparatus implemented for a two-hundred (200) pL fluid application for about three-hundred (300) dots per inch (dpi).
In its basic aspects, the present invention provides a method for heat treating print media, including the steps of: establishing at least two, discrete, temperature zones on a platen in a media transit axis; and transporting the media in the media transit axis in contact with the platen.
In another aspect, the present invention provides a method for anisotropically heat treating a print media to be printed with a wet colorant during transport from an input supply to an output, including the steps of: maintaining a substantially uniform temperature profile across a colorant receiving axis; and providing a plurality of temperature regions along the media transport axis wherein each of said temperature regions has a single said substantially uniform temperature profile.
In another aspect, the present invention provides a method of distributing heat anisotropically across an ink-jet platen including: substantially uniformly heating a pre-printing zone of a media transit axis to a first temperature for pre-conditioning print media; substantially uniformly heating a printing zone of a media transit axis to a second temperature for printing on the print media; and providing a cool down zone between said pre-printing zone and said printing zone.
In another aspect, the present invention provides an anisotropically heated platen apparatus, including: a heated ingress region for receiving print media superjacently thereon; and a heated printing region downstream of the ingress region for sequentially receiving the print media, wherein said ingress region is at a first predetermined temperature and said printing region is at a second predetermined temperature, said ingress region and said printing region are substantially isolated thermally such that thermal exchange therebetween is minimized.
Another aspect of the present invention is a liquid colorant print media platen apparatus, including: sequentially in a media transit axis, a first region substantially uniformly heated in orthogonal axes to a first predetermined temperature for preconditioning print media, a second region that is unheated, and a third region substantially uniformly heated in like orthogonal axes to a second predetermined temperature for depositing said colorant on the print media, wherein anisotropic print media heat conditioning occurs on said platen.
In another aspect, the present invention provides an ink-jet hard copy apparatus, having a known manner means for inducing a vacuum force, including: at least one ink-jet writing instrument for depositing ink drops onto pixels of an adjacently positioned sheet of print media; adjacent to said writing instrument, a print media platen including a thick film transport surface, having vacuum ports in a first array; mounted to the platen, individually selectable heaters in a second array interspersed with said first array; a perforated print media transport belt for sliding across said surface for carrying said sheet via vacuum adhesion sequentially from an input position to a position of being adjacently positioned to said writing instrument to an output receiver; and a controller connected to said heaters for forming at least two segregated, anisotropic, print media heating regions on said platen surface.
Some of the advantages of the present invention are:
it provides a level x-axis temperature profile regardless of media size, and media position
providing a flat x-axis temperature profile promotes uniform ink drying to avoid hue shift;
providing a flat x-axis temperature profile avoids problems with edge scalloping and cockle;
providing a flat x-axis temperature profile avoids over heating conditions with respect to the pens, platen and media;
providing good power density control on the y-axis provides an improved temperature profile between pre-heating, cool down, print zone heating, and post-printing heating;
providing good power density control on the y-axis provides fast response time between temperature changes in a particular heating or cooling zone;
providing a limited z-axis thermal conductivity prevents heat losses from x-axis and y-axis heating, resulting in lower power consumption;
providing a limited z-axis thermal conductivity allows a faster response time due to low thermal mass design;
it reduces thermally induced impacts on ink-jet pen performance; and
it provides a method and apparatus wherein temperature profiles are independent of load, thus providing a safety mechanism against overheating.
The foregoing summary and list of advantages is not intended by the inventors to be an inclusive list of all the aspects, objects, advantages, or features of the present invention nor should any limitation on the scope of the invention be implied therefrom. This Summary is provided in accordance with the mandate of 37 C.F.R. 1.73 and M.P.E.P. 608.01 (d) merely to apprise the public, and more especially those interested in the particular art to which the invention relates, of the basic nature of the invention in order to be of assistance in aiding ready understanding of the patent in future searches. Other aspects, objects, advantages, and features of the present invention will become apparent upon consideration of the following explanation and the accompanying drawings, in which like reference designations represent like features throughout the drawings.